This invention relates to the removal of wellhead assemblies and, more particularly, to the cutting of well casing below a wellhead to enable removal of the wellhead.
When an oil or gas well is to be abandoned the usual procedure adopted is to plug the well with a suitable cement composition test, the integrity of the plug, and then remove the wellhead assembly. On land, the wellhead assembly can be removed by standard construction techniques and in general the casing immediately below the wellhead will be cut off several meters below ground level to allow reinstatement of the well site. This technique cannot satisfactorily be applied to subsea wells.
It is accordingly known, in the case of a subsea well, to plug the well bore with cement and then to detonate an explosive charge within the well casing slightly below the level of the wellhead proper in order to cut the casing at that point and free the wellhead assembly for removal. This technique is, however, unsatisfactory since the portions of the wellhead removed after explosive cutting are generally damaged and not suitable for re-use.
There is disclosed in EP-A-436706 a technique for cutting and removal of a wellhead by means of a rotating cutting tool mounted at the end of a drill string which is guided through the wellhead by means of a recovery tool. Because this solution requires the use of a drill string it is only applicable to wellheads which are located beneath an appropriate drilling rig. Although, in theory, this method could be applied to a wellhead which is not located beneath a drilling rig to bringing a drilling barge over the wellhead and running the necessary drill string from the drilling barge, such a technique would be prohibitively expensive given the high cost of operating a drilling barge.